The United States Postal Service (USPS) regulates by size and mailing cost the envelopes that it accepts for delivery. Size applies to weight, height, length, thickness and shape. These criteria affect the mailing cost. Additionally, the size will determine whether the envelope can be handled through the mail-sorting machines or if it must be sorted manually and subjected to a surcharge.
To send a letter as first class mail, it must meet the regulations of the USPS, or it will be subjected to additional fees. The dimensions of a first-class letter must be at minimum 3 1/2 inches by 5 inches and .007 inches thick. At maximum, the measurements are 6 1/8 inches by 11 1/2 inches long and 1/4 inch thick. The maximum weight of a first-class is 3.5 ounces.
Envelopes classified as first-class large exceed one or more of the maximum dimensions for first-class. The weight limit in the large category is 13 ounces. Regulations for maximum size are 12 inches by 15 inches and 3/4 inches thick. Parcel-post rates apply.
First-class thick envelopes must weigh 13 ounces or less, and the weight determines mailing cost. Thick envelopes above the weight limit are categorized as parcel post, media mail or priority mail rather than first-class.
Unusual Shapes and Sizes
Postal regulatons permit envelopes that are of unusual sizes and shapes, but they require additional postage if the sorting machines cannot process them.